drm/i915/evict: Always switch away from the current context

Currently execlists is exempt from emitting a request to switch each
ring away from the current context over to the dev_priv->kernel_context
(for whatever reason, just under execlists the GGTT is unlikely to be as
fragmented, however the switch may help in some extreme cases). Extract
the switcher and enable it for execlsts as well, as we need to do so in
a later patch to force the context switch before suspend. (And since for
that switch we explicitly require the disposable kernel context, rename
the extracted function.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468590980-6186-1-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 1ec523d..cd1ccc4 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3525,6 +3525,7 @@
 int i915_gem_context_open(struct drm_device *dev, struct drm_file *file);
 void i915_gem_context_close(struct drm_device *dev, struct drm_file *file);
 int i915_switch_context(struct drm_i915_gem_request *req);
+int i915_gem_switch_to_kernel_context(struct drm_i915_private *dev_priv);
 void i915_gem_context_free(struct kref *ctx_ref);
 struct drm_i915_gem_object *
 i915_gem_alloc_context_obj(struct drm_device *dev, size_t size);